1. Field of the Invention
The technical field relates to a semiconductor device, a display device, a light-emitting device, and a method for manufacturing these devices. The technical field particularly relates to a semiconductor device including a thin film transistor (hereinafter also referred to as a TFT) using an oxide semiconductor.
2. Description of the Related Art
Thin film transistors (TFTs) in which a silicon layer of amorphous silicon or the like is used as a channel layer have been widely used as switching elements in display devices typified by liquid crystal display devices. Although thin film transistors using amorphous silicon have low field-effect mobility, they have an advantage that larger glass substrates can be used. In addition, there has been known a technique for forming a pixel portion and some of driver circuits on one substrate in an integrated manner in order to reduce manufacturing costs.
Moreover, attention has been recently drawn to a technique by which a thin film transistor is manufactured using a metal oxide with semiconductor properties and such a transistor is applied to an electronic device or an optical device. For example, it is known that some metal oxides such as tungsten oxide, tin oxide, indium oxide, and zinc oxide have semiconductor properties. A thin film transistor in which a light-transmitting semiconductor layer formed using such a metal oxide is used as a channel formation region is disclosed (e.g., see Patent Document 1).
Furthermore, a technique has been considered to increase the aperture ratio in such a manner that a channel layer of a transistor is formed using a light-transmitting oxide semiconductor layer and a gate electrode, a source electrode, and a drain electrode are formed using a light-transmitting conductive film (e.g., see Patent Document 2).
The increase in aperture ratio increases the light use efficiency, the reduction in power and size of display devices can be achieved. On the other hand, in terms of the increase in size of display devices and application of display devices to portable devices, a further reduction in power consumption as well as the increase in aperture ratio is required.
As a method for placing a metal auxiliary wiring for a light-transmitting electrode of an electro-optic element, there is known a method by which a metal auxiliary wiring and a light-transmitting electrode are placed to overlap with each other so that the auxiliary wiring is brought into conduction with the light-transmitting electrode above or below the light-transmitting electrode (e.g., see Patent Document 3).
A structure is known in which an additional capacitor electrode provided on an active matrix substrate is formed using a light-transmitting conductive film of ITO, SnO2, or the like and an auxiliary wiring formed using a metal film is provided in contact with the additional capacitor electrode in order to reduce the electrical resistance of the additional capacitor electrode (e.g., see Patent Document 4).
It is known that in an electric-field transistor including an amorphous oxide semiconductor film, a light-transmitting electrode formed from indium tin oxide (ITO), indium zinc oxide, ZnO, SnO2, or the like; a metal electrode formed from Al, Ag, Cr, Ni, Mo, Au, Ti, Ta, or the like; a metal electrode formed from an alloy containing any of the above elements; or the like can be used for a gate electrode, a source electrode, and a drain electrode, and two or more of such materials may be stacked to reduce the contact resistance or to increase the interface intensity (e.g., see Patent Document 5).
It is known that a metal such as indium (In), aluminum (Al), gold (Au), or silver (Ag); or an oxide material such as indium oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO), cadmium oxide (CdO), indium cadmium oxide (CdIn2O4), cadmium tin oxide (Cd2SnO4), or zinc tin oxide (Zn2SnO4) can be used for a source electrode, a drain electrode, and a gate electrode of a transistor including an amorphous oxide semiconductor and an auxiliary capacitance electrode, and the materials for the gate electrode, the source electrode, and the drain electrode may be the same or different from each other (e.g., see Patent Documents 6 and 7).